The present invention relates to subsea drilling of oil and gas wells and in particular, though not exclusively, to apparatus and method for a one trip deployment and drilling system to rapidly create exploration wells.
Current practice in subsea drilling is to firstly establish a well by drilling a surface hole at the sea floor. Conductor casing is cemented at the top of the well upon which is located a subsea wellhead. Thereafter a Subsea Blowout Preventer Stack (BOP) is run and latched to the wellhead. Subsequent casing strings are then suspended in the wellhead.
In many exploration wells drilled from floating vessels, the current trend is to use a BOP at the vessel rather than at the sea floor. This is done to reduce the number of trips required to the sea floor and reduce the complexity of the procedure. However, this arrangement is only acceptable for some situations where the vessel is moored in benign metocean conditions. Where there is a risk that the vessel may loose station over the well, especially where it is maintained on station over the well by dynamic positioning, a shut off device at the sea floor is being seen as a minimum requirement. The shut off device must be capable of providing rapid disconnection between the riser, attaching the well to the vessel, and the wellhead.
WO 02/088516 to Shell International Research Maatschappij B.V. describes a system for drilling a subsea well, comprising a surface BOP, a subsea BOP connected to the wellhead and a drilling riser there between. Drilling is achieved by running a drill string through the riser. The subsea BOP includes a selective disconnection system so that the riser can be disconnected in the event of the vessel/platform moving. While this system addresses requirement for a subsea shut-off device it has the inherent disadvantage in the time taken to establish a well as the wellhead is installed before the subsea BOP is located and connected to the riser. Finally the drill string is run through the connected riser.
Running a shut off device down to a subsea wellhead saves little time over current subsea drilling practices.
It is therefore an object of the present invention to provide a method of establishing a subsea well where the shut-off device is run-in with the drilling assembly so that the well is drilled and established on a single trip without the need to first drill and locate a wellhead.
A further object of the present invention is to provide a subsea shut off device which can be run on casing and released so that the casing can be used for drilling the well.